The synthesis of inorganic materials with controlled and complex forms has been facilitated through discoveries such as vesicle, micelle and liquid crystalline templating of silicates which provided inspiration to explore a range of templating strategies based on self-assembled molecular precursors, colloids, and biological templates and vessels. See C. T. Kresge et al., Nature 359, 710 (1992); J. S. Beck et al., J Am Chem Soc 114, 10834 (1992); S. Mann and G. A. Ozin, Nature 382, 313 (1996); C. J. Brinker et al., Adv Mater 11, 579 (1999); E. Pouget et al., Nat Mater 6, 434 (2007); C. L. Chen and N. L. Rosi, Angew Chem Int Ed 49, 1924 (2010); K. E. Shopsowitz et al., Nature 468, 422 (2010); C. Boissiere et al., Adv Mater 23, 599 (2011); B. T. Holland et al., Science 281, 538 (1988); B. Hatton et al., Proc Natl Acad Sci USA 107, 10354 (2010); A. Stein et al., Chem Mater 20, 649 (2008); O. Paris et al., MRS Bull 35, 219 (2010); D. Van Opdenbosch et al., J Mater Chem 26, 1193 (2011); and K. J. C. van Bommel et al., Angew Chem Int Ed 42(9), 980 (2003). A driving force for these efforts is the many complex inorganic structures found in nature. An oft-cited example is the hierarchical composites built by silica condensing microorganisms such as diatoms, which have generated substantial scientific interest for over a century. See P. Fratzl and S. Weiner et al., Adv Mater 22, 4547 (2010). Diatoms display complex 3D architectures with great structural control over nano- to millimeter length scales. However, despite some success toward elucidating mechanisms of diatom biomineralization, the in vitro synthesis of 3D diatom-like forms has remained elusive. Diatom silica has found numerous applications including as a chemical stabilizer, absorbent, filter medium, and fine abrasive, and the lack of synthetic analogues has facilitated recent investigations to employ diatom frustules as starting materials for shape-preserving chemical transformations into functional nanomaterials. See K. H. Sandhage et al., Handbook of Biomineralization: Biomimetic and bioinspired chemistry, 235 (2007); D. Losic et al., Adv Mater 21, 2947 (2009); and Z. Bao et al., Nature 446, 172 (2007). Given the potential of this biosilica, it would be desirable to be able to wield control over the silica structure in order to achieve broader applicability; however, strategies to direct diatom morphology using chemical and genetic approaches has proven challenging. See M. Hildebrand, J Nanosci Nanotechno 5, 146 (2005); H. E. Townley et al., Nanotechnology 18, 295101 (2007); and N. Kroger, Curr Opin Chem Biol 11, 662 (2007). Therefore, an ability to generate cell frustules from more malleable templates such as mammalian cells would provide greater access to natural and engineered cell heterogeneity—both structure and function—to be exploited in the design of complex materials.
To these ends, biomineralization by silica condensing microorganisms offers key lessons. The discovery of biogenic peptides that catalyze silica condensation subsequently has motivated the extensive investigation of the interaction of natural and synthetically-derived peptides and proteins with silica and its precursors. See N. Kroger et al., Proc Natl Acad Sci USA 97, 14133 (2000); N. Kroger et al., Science 286, 1129 (1999); J. N. Cha et al., Proc Natl Acad Sci USA 96, 361 (1999); E. Pouget et al., Nat Mater 6, 434 (2007); N. Kröger et al., Science 298, 584 (2002); T. Coradin et al., Colloids Surf B 29, 189 (2003); A. Bassindale et al., J Mater Chem 19, 7606 (2009); C. Gautier et al., Colloids Surf B 65, 140 (2008); M. Dickerson et al., Chem Rev 108, 4935 (2008); and S. V. Patwardhan et al., Chem Commun 9, 1113 (2005). Identification of silica associated biomolecules such as long-chain polyamines and the silaffin peptides has led to a general understanding of the tenets by which macromolecules control polymerization of silica precursors into silica assemblies. See N. Kroger et al., Proc Natl Acad Sci USA 97, 14133 (2000); N. Kroger et al., Science 286, 1129 (1999); and M. Hildebrand, Chem Rev 108, 4855 (2008). However, silica morphogenesis at the meso- and micro-scales must involve both transport of soluble silica precursors and their directed deposition by biomolecular templating or structural elements. See B. Tesson and M. Hildebrand, PloS one 5, e14300 (2010); E. Brunner et al., Angew Chem Int Ed 48, 9724 (2009); and A. Scheffel et al., Proc Natl Acad Sci USA 108, 3175 (2011). Likely, these larger scale molecular assemblies direct the assembly of silica building blocks, formed in the silica deposition vesicle (SDV), into complex structures.
Therefore, Khripin et al. recently examined whether synthetic 3D protein scaffolds could direct/template silica deposition provided the appropriate silica precursors and chemical conditions. See C. Y. Khripin et al., ACS Nano 5, 1401 (2011). They showed that microfabricated protein hydrogels could template silica volumetrically into mechanically stable, nano- to micro-scale biocomposites with user-defined 3D features identical in size and shape to those of the template. These features were preserved following removal of the organic component to form a porous silica replica. Importantly, proteins of diverse properties (e.g., isoelectric point; pI) directed silica condensation under identical solution conditions (100 mM silicic acid, pH 3), which is somewhat contrary to the generally held understanding that cationic species (e.g., proteins with pI>7) are required for biogenic silica deposition. See A. Bassindale et al., J Mater Chem 19, 7606 (2009). These protein hydrogels are highly concentrated (>40% protein by wt vol−1), producing a locally crowded 3D molecular environment, which acts to capture and concentrate silica precursors (mono-, oligo-silicic acid, and nanoparticles) via hydrogen bonding and other non-covalent interactions, promoting their further condensation and conversion to covalently bonded siloxane replicas.
However, a need remains for a method of directed silica condensation in naturally crowded molecular environments, such as cells, under similar conditions.